1. Field of the Invention
The present invention relates to an extreme ultraviolet (EUV) light source apparatus to be used as a light source of exposure equipment.
2. Description of a Related Art
In recent years, as semiconductor processes become finer, photolithography has been making rapid progress toward finer fabrication. In the next generation, microfabrication at 70 nm to 45 nm, further, microfabrication at 32 nm and beyond will be required. Accordingly, in order to fulfill the requirement for microfabrication at 32 nm and beyond, for example, exposure equipment is expected to be developed by combining an EUV light source for generating EUV light having a wavelength of about 13 nm and reduced projection reflective optics.
As the EUV light source, there are three kinds of light sources, which include an LPP (laser produced plasma) light source using plasma generated by applying a laser beam to a target (hereinafter, also referred to as “LPP type EUV light source apparatus”), a DPP (discharge produced plasma) light source using plasma generated by discharge, and an SR (synchrotron radiation) light source using orbital radiation. Among them, the LPP light source has advantages that extremely high intensity close to black body radiation can be obtained because plasma density can be considerably made larger, that the light emission of only the necessary waveband can be performed by selecting the target material, and that an extremely large collection solid angle of 2π steradian can be ensured because it is a point light source having substantially isotropic angle distribution and there is no structure such as electrodes surrounding the light source. Therefore, the LPP light source is considered to be predominant as a light source for EUV lithography, which requires power of more than several tens of watts.
In the LPP type EUV light source apparatus, EUV light is generated on the following principle. That is, by supplying a target material into a vacuum chamber by using a nozzle and applying a laser beam to the target material, the target material is excited and turned into plasma. Various wavelength components including extreme ultraviolet (EUV) light are radiated from the plasma generated in this manner. Then, a desired wavelength component (e.g., 13.5 nm) among them is selectively reflected and collected by using a collector mirror, and outputted to an exposure unit (projection optics). For example, as a collector mirror for collecting EUV light having a wavelength near 13.5 nm, a mirror having a reflecting surface on which molybdenum (Mo) and silicon (Si) thin films are alternately stacked is used. Typically, the number of stacked Mo/Si thin films is from sixty to several hundreds.
As a related technology, Japanese Patent Application Publication JP-P2006-108686A discloses a lithography apparatus for applying EUV light to a virtual light source point of projection optics in alignment with an optical axis of the projection optics by providing an oblique incidence mirror within an EUV light source apparatus (radiation unit).
However, in JP-P2006-108686A, loss in EUV light intensity is caused by providing the oblique incidence mirror. Generally, the reflectance of EUV light by a mirror is about 60%, and therefore, the use efficiency of EUV light becomes lower by about 60% at each time one mirror is added.
Further, U.S. Patent Application Publication US 2006/0146413 A1 discloses a lithographic apparatus for applying EUV light to a virtual light source point of projection optics in alignment with an optical axis of the projection optics by placing an EUV light source apparatus obliquely relative to the direction of gravitational force. According to US 2006/0146413 A1, the number of reflection mirrors is reduced by one than that in JP-P2006-108686A, and therefore, the use efficiency of EUV light can be improved.
However, when the EUV light source apparatus is obliquely placed as in US 2006/0146413 A1, detachment of a chamber or a part of the chamber, movement to a maintenance area, and highly accurate placement relative to the projection optics become difficult in the case of maintenance of the EUV light source apparatus.